Determination of Five Alternaria Toxins in Tomatoes by UHPLC-MS/MS with Purification by QuEChERS

A portion of homogenized pulp of tomato sample (5.0 g) was place in 50 mL polypropyrene centrifugation tube and extracted ultrasonically with 10 mL of acidified acetonitrile (4.0 mL of formic acid was added to 996 mL of acetonitrile) for 30 min, to dissolute the 5 alternaric toxins (i.e. AOH, AME, ALT, TEN and TeA) into acetonitrile. Sodium chloride (0.5 g) and anhydrous magnesium sulfate (5 g) were added and centrifuged at high speed for 5 min to have the extract dehydrated. An aliquot of 1.5 mL was taken from the extract, and purified by swirling with 25 mg of C₁₈ for 1 min to remove interfering co-extracted substances, e.g. some pigments and etc. An aliquot of 0.5 mL of the purified extract was transferred to a centrifugation tube, and after adding 0.5 mL of 1 mmol·L^-1 (NH₄)HCO₃ solution, centrifuged at high speed for 5 min. The supernatant was used for UHPLC-MS/MS analysis. ACQUITY UPLC BEH C₁₈ chromatographic column was chosen as stationary phase, and mixtures of 1 mmol·L^-1 (NH₄)HCO₃ solution (A) and methanol (B) in various proportions were used as mobile phases in gradient elution. In MS/MS determination, electrospray ionization with switching scanning of ESI⁺ and ESI^-, and the mode of MRM were adopted. Standard curves for the 5 alternaric toxins were prepared by the matrix matching method, and linear relationships were found in the same range of 0.5 to 100 μg·L^-1, with values of detection limits (3S/N) in the range of 0.6 to 3.0 μg·kg^-1. Using blank sample as matrix, tests for recovery were made by standard addition method, giving values of recovery in the range of 81.6% to 115%, and values of RSDs (n=6) were in the range of 2.1% to 11%.